The human body was not designed for sitting. Humans are designed to ambulate on two legs with the makeup of the skeletal support within the body designed for walking. That being the case, humans do spend a lot of time sitting and a significant number are not able to stand or walk due to accident, disease or age related limitations. People that sit for a large portion of time during the day may require specialized seating to provide increased comfort, controlled posture or protection from the development of decubitus ulcers (also known as bed sores or pressure sores).
Relevant Anatomy
FIG. 1A is a side view of a prior art seated person showing primary anatomical areas of the pelvis supporting the person while sitting. It show the primary anatomical areas of the pelvis that are important in describing how prior art and the current cushions function. There are several primary areas that are important relative to support of the pelvis and the upper torso of a person when in a seated position. The areas that are in contact with the seat cushion are the most important for this discussion. They are formed by a combination of the skeletal components and are of course surrounded by layers of soft tissue resulting in the familiar shapes of the buttocks and thigh.
The skeletal components most associated with supporting the body in a seated posture include the ischial tuberosities 101, greater and lesser trochanter 102 (at the hip Joint) and the long bone of the femur 103. The long bone of the femur 103 and trochanter 102 form the trochanteric shelf 104, an ideal place to shift load for pressure relief at the ischials 101 or coccyx 108 and to also improve lateral stability for the pelvis 100.
The first areas of concern are the two ischial tuberosities (ITs) 101. The IT 101 area of the pelvis 100 is the lowest point of the pelvis 100 when in a seated position. Viewed from the side, the ITs 101 are lower than the hip joint 105. In the average adult, the distance between the lowest point of the ITs 101 and the lowest part of the hip joint 105, the trochanter 102, is approximately 40 mm (1.57″). In addition to being lower, the ITs 101 have very sharp pointed contours. When in the seated posture with the feet supported on the floor, or on wheelchair footrests and the arms supported on armrests, the buttocks 106 and posterior thigh 107 will support approximately 65% of a person's body weight. As an example, a 200-pound person will have 130 pounds of weight distributed on the buttocks and posterior thigh with the peak pressures centered on the IT 101 area. Approximately 80% of all pressure sores for wheelchair users occur at the ischial tuberosities 101.
Another area of possible contact in the seated position is the sacrum and coccyx (tailbone) 108. The coccyx 108 is another sharp bony prominence that is not ideally suited for significant weight bearing and is also an area of increased risk for pressure sores. The coccyx 108 is higher than the ischials so the risk of pressure sores there is not as high as at the ITs unless the person sits in a “slouched” posture, but the risk is still significant.
A further concern is lateral stability of the pelvis 100. The spine 110 has a normal natural curvature at which the muscles supporting it need to do the least amount of work as shown in FIG. 1B, where a user is sitting on a cushion as in present embodiments (i.e., FIG. 1B is not prior art). This normal curvature is generally found when the person is walking with proper posture, standing up straight, or sitting up straight. However, all people tend to slouch or relax their posture at least slightly upon sitting down. As seen in FIG. 1A, this causes pelvic retrusion, where the pelvis 100 rotates slightly backward, causing the bottom of the pelvis 100 to move in an anterior direction, the top of the pelvis 100 to move in a posterior direction, or some combination of both movements. Since the spine 110 is attached to the pelvis 100, this pelvic retrusion causes the spine 100 to straighten and undergo a change in alignment of various vertebrae 111 away from the normal curvature of spine 110. As a result, muscles react between vertebrae in the spine, activating to urge the vertebrae back toward normal alignment. This muscle activation lasts the entire time the misalignment persists. The muscles thus must work harder to support the spine in this misaligned position, leading to muscle fatigue. The muscles may also experience further strain due to pressure exerted between misaligned vertebrae. The muscle fatigue and strain resulting from misalignment can lead to substantial lower back pain.
Prior Art Cushion Designs
Prior art wheelchair seat cushions come in a wide variety of designs, from a simple piece of polyurethane foam to very complex cushions with multiple density foams, foam and flexible gel layers or fluid bladders (air and/or viscous fluid). However, two primary design considerations are common to all cushions regardless of specific variety: heat buildup and pressure distribution.
Heat build-up in cushions is a design consideration because the support medium and cover materials used in wheelchair seat cushions may act as good insulators. The human body is warmer than average room temperature creating a situation where the heat of the body starts to warm the cushion when a person sits down. Since the cushion acts like an insulator, the heat is deflected back up to the body creating a rise in skin temperature. In a room at a customary ambient temperature of approximately 22° C. (72° F.), average skin temperature is about 24° C. Skin temperature at the seat cushion interface usually reaches 35°-37° C. in 60-120 minutes. As skin temperature increases to around 31° C. the body responds by increasing sweating in an effort to control heat buildup and maintain a constant core temperature. The point at which the body triggers this sweating is called the perspiration threshold. Moisture is caused by the skin reaching the perspiration threshold, triggered by heat.
Heat build-up and sticking clothing can be annoying, but for most people, it does not pose a serious health risk. However, for people that use wheelchair cushions, heat build-up is a primary factor for increased risk of developing pressure sores. The top three contributing factors are peak pressure at areas of high risk, heat, and moisture. Pressure applied to the skin and soft tissue closes off the capillaries and the soft tissue can die from lack of oxygen and/or nutrients. Moisture softens the skin and makes it more susceptible to physical damage. Heat causes a rather dramatic increase in cellular metabolism. As skin temperature increases 1° C., the metabolic demand increases 10%. The increase in metabolism means that the cells need more oxygen as the temperature increases and the soft tissue can die from lack of oxygen. Since skin temperature dramatically affects skin integrity, it is very important to prevent skin temperature build-up in wheelchair cushions.
To address the pressure issue, most cushions support the body by allowing the body mass to sink into or immerse into the cushion. The first points of contact are the ischials. Cushions that are successful in providing comfort and decreasing the risk of pressure sore development thus all have a common design requirement of redistributing pressure away from the sharp boney prominences of the ischials and shifting those pressures to the rest of the seated support surface at the hips and trochanteric shelf.
There are three ways in which a cushion can support a person. The most common is that the shape of the cushion changes with the applied load. The vast majority of cushions work in this way. Cushions made from resilient foams will compress allowing the body to sink into or immerse into the cushion. This allows the cushion to change shape and adapt to the user. Some cushions have a fluid interface with the user. In this configuration, the fluid will move out of the way of high pressure and flow to areas of low pressure as it attempts to equalize support.
The key to the function of these cushions is that the material used to fabricate the cushions has the ability to change shape under load. The foam compresses or the fluid moves. When foam is compressed the elastic properties of the foam offer some resistance to compression as it changes from a flat sheet to a contoured surface. The resilient nature of the foam behaves like a series of springs standing on their ends, much like a mattress is constructed. As load is applied to a foam wheelchair cushion the first “springs” that would be compressed would be the ones under the IT areas and they would compress the furthest as load is applied over the entire cushion surface. Coil springs increase resistance the further they are compressed. The spring-like quality of polyurethane foam responds the same way. The pressure required to compress the foam increases as the foam is compressed. Since the foam is compressed the most under the ischials, the pressure is greater at those areas.
Another way to achieve the same type of pressure distribution and comfort is to design the cushion with a fluid interface. A fluid interface could either be a gas or liquid. Both materials are fluid in while different in physical properties. It is the nature of a fluid to move away from areas of high pressure and move to areas of low pressure. This allows the fluid cushion interface to allow immersion but also to provide greater levels of envelopment as the cushion forms to the shape of an object pushing against it. Cushions fabricated with multiple air bladders may have all of the air bladders interconnected. When a person sits on such a cushion, the air (gaseous fluid) is moved away from areas of high pressure and travels to areas of low pressure. This tends to equalize the pressure over the complete seating surface area and reduces peak pressure at areas of high risk. Fluid cushions that use a liquid instead of a gas follow the same laws of physics and will also move away from areas of high pressure and fill in areas of low pressure. Due to the higher viscosity of most fluids as compared to gases, liquid fluid cushions tend to adapt to the shape of the user slower than air filled cushions. This may improve stability, but the pressure relief principles are the same.
A second type of wheelchair cushion combines the resilient materials (foam or fluids) with a cushion shape that is pre-contoured to match a generic anatomical shape of a seated person. As an example, when a person sits on a soft moldable surface like sand or snow and then carefully gets up, there will be an imprint in that soft substrate that represents a normal anatomical shape. The contours will be lower underneath the IT area and will round upwards around the buttocks and will have two elongated troughs where the surface was compressed by the thighs. One of the ways to reduce the peak pressure build up under the IT area and to provide more comfort overall is to pre-contour the cushion so that the cushion does not have a flat top surface. This allows the cushion supporting the body by starting out with a shape that closely matches a general human anatomy. A cushion is pre-contoured if it is fabricated with a top shape that mimics the same general shape of the buttocks and thighs that is found in a seated person. When a cushion has this generic pre-contoured configuration, the support medium does not have to compress as much to match the shape of the user and pressures can be redistributed to the trochanteric shelf and away from the ischials more efficiently.
A related method for transferring load away from the areas of peak pressure and improving pressure distribution and comfort is to fabricate the cushion from a variety of materials that provide a firmer surface underneath the trochanteric shelf and a softer surface underneath the ischial area. Using this multi-Density foam technique is rather common in the wheelchair cushion industry. This can be done with a flat or precontoured cushion but still relies on the same principles of cushion support outlined above.
A third method of redistributing pressure is to fabricate the cushion to the exact shape of the individual user. In this technique, the person is positioned on a cushion that has been molded to their specific shape and posture. There are several techniques to accomplish this but the end result is that the cushion and person have the same shape. Because the dimensional differences between the ischials and trochanteric shelf are addressed and there is a lot of surface area bearing load, there is usually little need for the cushion to change shape or allow immersion to accommodate the boney prominences of the user. This technique is very good, but the process can be time consuming and very expensive and is prone to fitment problems if the user grows or changes shape by gaining or losing weight.